UPDATE: According to CBS News, two U.S. law enforcement officials confirmed the attack was carried out by a suicide bomber. The suspect reportedly took the Tube to Victoria Station and set off the device near the ticket area as concertgoers began exiting the building. British authorities are investigating reports that the device was a "nail bomb" and have found ball bearings at the scene of the attack.

We have been treating this as a terrorist incident and we believe that while the attack last night was conducted by one man, the priority is to establish whether he was acting alone or as part of a network. The attacker, I can confirm, died at the arena. We believe the attacker was carrying an improvised explosive device, which he detonated, causing this atrocity.

Witnesses told The Guardian that they heard the explosion after the concert had ended, and one eyewitness said that he saw "five people with blood all down them." According to The Telegraph, British Transport Police have confirmed that the explosion came from "within the foyer area of the stadium at 10:30 pm" local time. Footage showed police and emergency vehicles assembling outside the area. The nearby Manchester Victoria train station has been evacuated, according to The Telegraph, and all upcoming trains cancelled.

The circumstances and cause of the explosion haven't yet been determined, so it's impossible to say whether it was the result of a terrorist attack, a technical malfunction, or any number of other possible causes. Sky News reported that police are treating it as "a possible terrorist incident," but they haven't yet confirmed any motive.

Tim Farron, a member of British Parliament and leader of the Liberal Democrats, released a statement referring to the incident as "an attack on innocent people"; again, this claim has not been confirmed by police or any other authorities.

"I was in the toilet and heard a loud bang just after the concert had finished and people had started to leave," another eyewitness told The Guardian. "The bang echoed around the foyer of the arena and people started to run. I seen people running and screaming towards one direction and then many were turning around to run back the other way."

It may be a while until we learn with any certainty what exactly happened at Manchester Arena. Authorities haven't made any announcements one way or the other yet, other than that they're investigating the possibility that it's a terrorist attack. Until there's confirmation, it's best to hold off on drawing any major conclusions about what caused the explosion.